1. Field of the Invention
The present invention generally relates to a method of analyzing a laminated object and a system for analyzing a laminated object. Particularly, the present invention relates to non-destructively analyzing laminated objects such as composite blades for wind turbine installations.
2. Description of the Related Art
In general, defects in material of a laminated object induce strain anomalies resulting in structural weaknesses which may negatively affect the quality of a laminated object. In particular tolerances and/or problems appearing in one or more process steps of a manufacturing processes for manufacturing laminated objects represent sources of material defects, and therefore strain anomalies, and compliance with quality standards requires examination of manufactured products in this respect to approve load capacitance with regard to future applications.
Depending on the type and size of manufactured products to be examined, various techniques are used to detect weaknesses in the employed materials. For example, measuring techniques involving ultrasound, radiometry, inductive or capacitive measuring methods and the like allow for examining the material on the presence of defects in small measuring fields. Particularly with regard to large surface laminated objects, e.g. wind turbine blades, aircraft wings etc, according techniques are time-consuming because when checking a large surface with a high degree of reliability, the entire object has to be divided into small measuring fields, where both, reliability and measuring time are directly related to the number of single measuring fields, generally resulting in long overall measuring times. On the other hand, optical inspection techniques of large surfaces allow for realizing fast measuring techniques involving interferometric measuring systems such as for example speckle interference techniques or shearography techniques.
Wind turbine blades for wind power installations are large-surface aerodynamically designed structures that are constructed primarily of fiberglass or carbon fiber reinforced composite materials. The manufacturing of these large composite structures is a difficult process that is normally performed with skilled manual labor. Particularly during the manufacturing of blades for wind turbine power installations, fabrication flaws such as delamination between layers of composite material, wrinkles or waviness between layers of composite material, lack of bond or debonds between bond lines of structural elements and leading and trailing edges of blades and wrinkled or bent fibers of fibered blade materials may appear, for instance, due to complexities in the blade manufacturing process. With their large sizes, wind turbine blades are very difficult to inspect while, on the other hand, the possible flaws or damages are very small in size in comparison with the whole blade. As many fabrication flaws are not even visible to the surface, visual examination of the surface may not be considered as providing an effective testing method for detecting fabrication flaws and assessing the quality of manufactured blades.
Document WO 2012/00337282 describes the use of a long range shearography camera for imaging deformation derivatives at great distances. A blade stressing mechanism and laser interferometry is performed in order to detect defects present in the surface of wind turbine blades.
However, known shearography methods do not allow to unambiguously detect all relevant defects because, on the one hand, washed out defects may not appear in measurements, these defects representing “false negatives”. On the other hand, artifactual signals may indicate the presence of defects where no defect exists, these signals representing “false positives”. In consequence, severe defects may be overseen or an examined object may be assessed as defective although no serious defect is present.
Document EP 1 061 332 A2 describes an apparatus for performing electronic shearography comprising a video camera for visualizing sequential shearogram images so as to identify defects as dynamically changing objects in an animated visualization of a sequence of shearograms.
In view of the above description it is desirable to provide a method of analyzing a laminated object and a system for analyzing a laminated object that allow to identify type, severity and position of material defects.
It is an object of the present invention to provide a method of analyzing a laminated object and to provide a system for analyzing a laminated object which overcomes the above-discussed draw-backs of the state in the art.